Ask the Attorney: Responsibility after dog bite

Atty Haymond,
We were doing some spring clean up on our lawn when a dog from our neighborhood got loose and attacked our dog. In the process of breaking them up,
my 9 year old son got bit as well.
The neighbors feel terrible, but since our dog was in our invisible fenced in yard, I don't think we should be responsible for the vet/doctor bills and my son is still upset about it!
Any advice would be appreciated,
Alice R